In this thesis, high temperature superconducting Y 1-x Fe x Ba 2 Cu 3 O 7-d samples with 0 ?x?0.1 were prepared by the conventional solid-state reaction. The X-ray diffraction pattern analysis shows that all the samples are single phase up to x=0.1 and no impurity phases are detected. All samples have an orthorhombic structure with Pmmm symmetry. Lattice parameters and unit cell volume of samples were determined by Rietveld refinement method which indicate an increase in the lattice parameter c with increasing Fe concentration up to 0.4%. Also, in-plane Cu(2)-O(2) bond lengths were found to be decreasing with increasing Fe doping up to an optimal concentration of 0.004. In other words, the CuO 2 planes become more distorted and hence charge carriers may have better chances of traortation, so it can be considered as an evidence for the increasing T c up to the optimal doping level. By increasing the Fe doping concentration to more than the optimal level, the Cu(2)-O(2) bond lengths start to increase, and cause CuO 2 planes to flatten. The flattening of CuO 2 planes is attributed to the decrease in the transition temperature. The real part of magnetic susceptibility versus temperature shows sharp drops from normal to diamagnetic superconducting state which is due to intra-grain transition. By analyzing the susceptibility data, it is evident that the superconducting transition temperature increases up to the optimal doping level, and then starts to decrease as the doping concentration increases. The observed variations of T c can also be detected in the imaginary part of the magnetic susceptibility curves. Furthermore, the peak position of the imaginary part starts to rise, resulting in sharper peaks, as the doping concentration is increased to the optimal value. The increased peak positions are signatures of enhanced flux pinning up to the optimal doping. By increasing the doping level, the imaginary peaks show lower and wider peaks which means that the flux pinning is affected negatively. The electrical resistivity measurements show nearly linear temperature dependence in the normal state. The normal state resistivity of the samples decrease with increasing Fe content up to the optimal concentration, and then it increases as the doping level is increased. Increasing of transition temperature with increasing Fe content in the Y site up to an optimal value is also confirmed by the electrical resistivity transition temperatures, and is clearly inconsistent with the Abrikosov-Gorkov theory.
